Recently, an orthogonal frequency division multiplexing (OFDM)/multiple-input multiple-output (MIMO) system has attracted attention as a broadband wireless mobile communication technology. Especially, the MIMO technology can increase in proportion to the number of antennas spectral efficiency which is difficult to achieve an existing single-input single-output (SISO) communication technology.
Hereinafter, the OFDM/orthogonal frequency division multiplexing access (OFDMA) system is described in brief.
The OFDM technique is a modulation scheme for multiplexing a high-speed transmission signal to a large number of orthogonal narrowband sub-carriers. In the OFDM technique, a data sequence having a high transmission rate is divided into multiple data sequences having a low transmission rate and these data sequences are simultaneously transmitted by using a plurality of sub-carriers. That is, the OFDM is a special form of a multi-carrier transmission technology for simultaneously transmitting data streams in parallel over a plurality of sub-channels.
The OFDM technique is one of multiplexing technologies in terms of simultaneously transmitting a high-speed original data sequence of one channel over multiple channels and also one of modulation technologies in terms of separately transmitting the data stream into multiple carriers. Waveforms of respective sub-carriers are orthogonal in a time axis but overlap in a frequency axis.
The OFDM technique may be used in a field of a wireless LAN, that is, 802.11a, 802.11g, 802.16, etc. and may be applicable to a field of mobile communication such as the fourth generation (4G) system beyond IMT-2000, a high bit-rate digital subscriber line (HDSL), an asymmetric digital subscriber line (ADSL), and a wireless asynchronous transfer mode (ATM).
There are many types of multiple access. The multiple access is a transmission scheme term representing that communication resources are shared, that is, a given time span, space, and a frequency band are commonly used by many users. Namely, a limited resource such as a transmission channel is evenly shared by a plurality of nodes efficiently. Meanwhile, the multiple access may be another form of multiplexing. The multiple access includes fixed assignment, dynamic assignment, wired assignment, etc.
The fixed assignment includes code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), and spatial division multiple access (SDMA). In exemplary embodiments of the present invention, an OFDMA technology among multiple access schemes is used.
In the OFDMA technology, all effective sub-carriers are not used by a single user. Instead, a set of the effective sub-carriers is differently split and assigned to a plurality of users.
In the OFDMA technology, resources are flexibly assigned to users in the same cell by using different sub-carrier sets. Moreover, a sub-channel having a part of the sub-carrier sets as one set resources is used by separately assigning resources by 2-dimension in time and frequency regions. Furthermore, it is possible to optimize a channel capacity by assigning sub-carriers on the frequency region according to a transmission speed demanded by a user. That is, sub-channels are dynamically assigned according to a user's demand and the number of sub-carriers assigned according to the transmission speed demanded by the user may be varied. The OFDMA technology can reduce interference within a cell since different sub-carriers are assigned to users.
Hereinafter, MIMO technology is described in brief.
The MIMO technology is a multi-antenna technology for performing high-speed communication by using multiple antennas. The MIMO technology includes a spatial multiplexing scheme and a spatial diversity scheme according to whether the same data is transmitted.
The spatial multiplexing scheme is to simultaneously transmit different data signals through multiple transmit-receive antennas. A transmitting side transmits different data signals through transmit antennas. A receiving side classifies the transmitted data signals by appropriate interference elimination and signal processing, thereby improving a transmission rate by as much as the number of transmit antennas.
The spatial diversity scheme is to obtain transmit diversity by transmitting the same data through multiple transmit antennas and is one of space-time channel coding techniques. The spatial diversity scheme can maximize a transmit diversity gain (performance gain) by transmitting the same data through multiple transmit antennas. However, the spatial diversity scheme is not a method of improving a transmission rate but a method of improving the reliability of transmission caused by a diversity gain.
The MIMO technology includes an open loop, such as Bell labs layered space-time (BLAST) and space-time trellis code (STTC), and a closed loop such as transmit antenna array (TxAA), according to whether channel information is fed back from a receiving side to a transmitting side.
FIG. 1 is a block diagram illustrating a structure of an OFDM/MIMO transmitter using a precoding technique.
In FIG. 1, the same reference numerals designate the same or like parts.
In a wireless communication system environment, a fading phenomenon may occur due to a multi-path time delay. A process of restoring a transmission signal by compensating for a signal generated by an abrupt environment variation due to fading is called channel estimation.
Referring to FIG. 1, transmitted data is restored at a receiving side with a channel estimation value estimated by an additional pilot channel (or a reference channel). Accordingly, when a transmitting side transmits an OFDM signal, it is important to configure the pilot channel for channel estimation so that the receiving side can restore a signal. To construct a MIMO channel, it is necessary to estimate a channel for each antenna and therefore a corresponding pilot signal is inserted for each antenna into a time axis and a frequency axis with a regular pattern.
In FIG. 1, coded signals inputted to an OFDM/MIMO transmitter is arbitrarily mixed in a scrambling module 101 and modulated in a modulation mapper 102 to signals suitable for transmission. The modulated signals are mapped to each transmission layer in a layer mapper 103 and beam-formed in a precoding module 104 to support multi-layer transmission in a MIMO system. For channel estimation, a resource element mapper 105 inserts corresponding pilot signals for respective antennas into the modulated signals with a regular pattern. An OFDM signal generator 106 generates OFDM signals, inserts the OFDM signals into the modulated signals, and then transmits the OFDM signals through antenna ports.
For channel estimation, a reference signal which is known to a transmitter and a receiver is used. The reference signal is also called a pilot signal. In mapping a general reference signal, a method of assigning the reference signal between data sub-carriers is used to increase an amount of data transmission.
In the MIMO system, a transmission power of the reference signal is transmitted by being boosted higher than a data sub-carrier in order to easily estimate a channel. In this case, however, a transmission power unbalance between antennas may be incurred. If power magnitudes of power amplifiers between antennas in a base station are different, a design cost may be increased and it may be difficult to perform channel estimation during data transmission.